Applications in the field of aviation power supply put forward higher requirements for efficiency and power density of power converters. LLC resonant converter has received extensive attention because it can realize soft switching. On the premise of ensuring the reliability of switching devices, improving the efficiency of LLC converter mainly depends on parameter optimization, thus it is necessary to accurately calculate the loss, especially for switching devices. Integrating the auxiliary power supply into the converter is beneficial to improve the power density of the module. Based on the Simplified Time Interval Analysis, this paper proposed a three degree-of-freedom parameter optimization method including inductance ratio, quality factor and dead time. A 270V/28V 800W power converter prototype is designed using the optimal results, which achieves 96.9% efficiency and 620W/inch3 power density with integrated auxiliary power supply.